It's VERY simple to correct the shape of the spoiler. All you need is masking tape, and a half round file. Mask off the fenders, and anything you don't wish to sand, and file away. It shouldn't take more then about ten minutes? to cut down the rear spoiler.
To Make the 3rd brake light. (SO much easier to say isn't it? ) Find a picture of the appropriate light, as straight on, as you can find. Adjust to apropriate size. Cut out picture, and white glue, to a piece of your favorite tape. Apply to the body in the appropriate spot. Take the back of a blade and carefully etch the outline, into the spoiler. Voila!
In the case of the 1986 SVO, it appears that there is actually a raised portion around the brake light... I'd apply the "decal" to some strip styrene, trim around the light, and make the 3rd light "ring", which could then be applied to the spoiler. (This is NOT a good picture to use, and only used for reference. Notice, that you can see the left mirror, but not the right? it is NOT straight on.)